BACKGROUND & AIMS: Indirect calorimetry (IC) is the gold standard to measure energy expenditure (EE) in hospitalized patients. The popular 30 year-old Deltatrac II(®) (Datex) IC is no more commercialized, but other manufacturers have developed new devices. This study aims at comparing for the first time simultaneously, two new IC, the CCM express(®) (Medgraphics) and the Quark RMR(®) (Cosmed) with the Deltatrac II(®) to assess their potential use in intensive care unit (ICU) patients. METHODS: ICU patients on mechanical ventilation, with positive end-expiratory pressure <9 cm H2O and fraction of inspired oxygen <60%, underwent measurements by the three IC simultaneously connected during 20 min to the ventilator (Evita XL(®), Dräger). Patients' characteristics, VO2 consumption, VCO2 production, respiratory quotient and EE were recorded. Data were presented as mean (SD) and compared by linear regression, repeated measure one-way ANOVA and Bland & Altman diagrams. RESULTS: Forty patients (23 males, 60(17) yrs, BMI 25.4(7.0) kg/m(2)) were included. For the Deltatrac II(®), VO2 was 227(61) ml/min, VCO2 189(52) ml/min and EE 1562(412) kcal/d. VO2, VCO2, and EE differed significantly between Deltatrac II(®) and CCM express(®) (p < 0.001), but not between Deltatrac II(®) and Quark RMR(®). For EE, diagrams showed a mean difference (2SD) of 25.2(441) kcal between Deltatrac II(®) vs. the Quark RMR(®), and -273 (532) kcal between Deltatrac II(®) vs CCM express(®). CONCLUSION: Quark RMR(®) compares better with Deltatrac II(®) than CCM express(®), but it suffers an EE variance of 441 kcal, which is not acceptable for clinical practice. New indirect IC should be further improved before recommending their clinical use in ICU.